1. Field of the Invention
The present invention relates to a transformer for use in a power supply and, more particularly, to a transformer which is for use in a power supply and which drives the backlight of a display panel of an electronic instrument, such as a liquid crystal TV or car navigational system, through an inverter.
2. Description of the Related Art
An electronic instrument such as a liquid crystal TV or car navigational system is equipped with a backlight using cold cathode fluorescent lamps or other electric-discharge lamps. To light up these electric-discharge lamps, inverter circuits have been used for obtaining high voltages from low-voltage DC power supplies.
Referring to FIG. 12, a transformer 110 using a magnetic core 115 has been proposed as a transformer for a power supply used to drive an inverter circuit in patent reference 1. The magnetic core 115 is made of ferrite cores. In the transformer 110, the magnetic core 115 is made of a combination of an I-shaped core 115a and a U-shaped core 115b. A primary winding 111 and a secondary winding 112 are wound around a bobbin 113 having a cylindrical portion 113a provided with a magnetic core insertion hole (not shown). The I-shaped core 115a is mounted in the magnetic core insertion hole. The U-shaped core 115b is located over the bobbin 113 as viewed in a direction orthogonal to the direction in which the windings are wound on the bobbin 113. The U-shaped core 115b is opposite to the I-shaped core 115a along a line located outside the magnetic core insertion hole in the bobbin 113.
However, in the aforementioned transformer 110 for the backlight, where the used switching pulses have a frequency lying in the audio frequency band, the whole transformer buzzes due to resonance phenomena of higher harmonics.
Patent reference 2 proposes a technique for reducing buzzing. In particular, as shown in FIG. 13, an EI-shaped magnetic core 215 has an E-shaped magnetic core 215b including a center pole of magnetic core 215b1. The center pole of magnetic core 215b1 is located opposite to an I-shaped magnetic core 215a. A magnetic gap 215g is formed between the center pole of magnetic core 215b1 and I-shaped magnetic core 215a. A spacer 217 whose both surfaces have been applied with adhesive 216 is mounted in the magnetic gap 215g. Thus, a coil component 210 is suppressed from buzzing.
Patent reference 1: JP-A-9-186024
Patent reference 2: JP-A-11-233348
As described above in relation to the related art, in the transformer 110 for a power supply, at least one of the cores 115a and 115b is inserted in the magnetic core insertion hole formed in the cylindrical portion 113a of the bobbin 113. If the cores 115a and 115b of the core subassembly are adhesively bonded respectively to the bases 113b of the bobbin 113 to prevent the core subassembly 115 from coming off, there is the problem that the level of sound pressure produced by the aforementioned buzzing is increased. In addition, the level of sound pressure produced by the buzzing varies widely from product to product. Confirmation of the level of sound pressure produced by each individual product has presented problems.
In the technique of patent reference 2, if the spacer 217 whose both surfaces have been applied with adhesive 216 is interposed in the magnetic gap 215g between the center pole of magnetic core 215b1 of the E-shaped magnetic core 215b and the I-shaped magnetic core 215a of the EI-shaped magnetic core 215 as described above in relation to the related art, the level of sound pressure produced by the buzzing tends to decrease but the level of sound pressure varies more widely among products. Consequently, confirmation of the level of sound pressure produced by each individual product has presented problems.
In the technique of patent reference 1, if the spacer whose surfaces opposite to the cores have been applied with adhesive is interposed between the cores in the backlight transformer 110 as described above in relation to the related art, it has been impossible to obtain the expected buzzing-suppressing effects. There is another problem that the magnetic coupling between the primary winding 111 and secondary winding 112 decreases.